Today, light-emitting elements (LEDs) are widely used as light sources in light source modules for use in liquid crystal display devices and lighting devices.
In liquid crystal display devices, a method of backlighting in which light from a light source module is shone on a liquid crystal layer from behind is quite common. There are roughly two methods of backlighting: in edge-lit backlighting, a light source module is arranged in a row along one or more of the top, bottom, left side, and right side of a light guide plate arranged directly behind a liquid crystal layer; in direct-lit backlighting, a light source module is arranged directly behind a liquid crystal layer. With either method, a large number of LEDs are needed to obtain a sufficient amount of light as a backlight. Also in use as a lighting device, a large number of LEDs are needed to obtain a sufficient amount of light as a lighting device.
In cases where a large number of LEDs are needed. LEDs are often spread among, and mounted on a plurality of circuit boards in other words, a light source module is composed of a combination of a plurality of circuit boards each having a plurality of LEDs mounted thereon. In liquid crystal display devices, the number of LEDs needed increases with the screen size. Thus, large-screen liquid crystal display devices (for example, those installed outdoors for large-screen digital signage) require a large number of LEDs, and require an accordingly large number of circuit boards on which to mount LEDs.
Adopting a design where a plurality of LEDs are driven separately for each circuit board leads to an increased number of power supply conductors, and hence a complicated wiring and cabling layout. To lessen the trouble, it is common to adopt a method in which a plurality of circuit boards are mechanically and electrically coupled together to form a channel (set) so that a plurality of LEDs are driven together for each such channel (see, for example Patent Document 1 identified below). A plurality of circuit hoards can be electrically coupled together, roughly speaking, in one of two electrical circuit configurations: a serially-connected and a parallel-connected configuration.
With a serially-connected configuration, the larger the number of circuit boards coupled together, the higher the voltage involved. This may pose problems in terms of product safety and the withstand voltage of components in the light source module. On the other hand, with a parallel-connected configuration, the larger the number of circuit boards coupled together, the higher the current that passes through an LED string when a defect occurs such that part of the LED string is short-circuited, possibly leading to smoking or ignition. Moreover, if a defect occurs such that part of an LED string remains open, although it has to be detected based on a change in voltage drop, the change in voltage drop is so small that it is difficult to accurately discriminate whether the change in voltage drop is ascribable to a mere measurement error or an open defect.
For these reasons, in a case where a large number of circuit boards are combined together, it is preferable to minimize the number of circuit boards that together form one channel (that are electrically coupled together).